We propose two aims in which Ig transgenic mice will be used to investigate the control of V gene expression in developing B-cells. The first aim deals with the expression of VH genes, which differs dramatically in B-cells of adult and fetal mice. In developing B-cells of adult mice, the expression of VH genes is known to be dependent on the efficiency with which different H (mu) chains pair with L chains (surrogate or conventional) and on the antigen-binding specificity that results when a given mu and L chain pair. Whether VH expression is similarly dependent on such cellular selection in developing fetal B-cells is not known. To address this issue, we will selectively breed two lines of IgL chain transgenic mice (V-kappa-8 and V-lambda-l mice) lacking the surrogate light (SL) chain and endogenous L(kappa) chains. In this model, V-kappa-8 or V-lambda-l chains will substitute for the SL chain. Nascent u chains will essentially have the choice of only a single L chain (V-kappa-8 or V-lambda-l) with which to pair and form a B-cell receptor (BCR). We will test V-kappa-8 and V-lambda-l mice for usage of different VH genes, differential mu-L chain pairing and for evidence of B-cell selection based on BCR specificity. Using a novel assay system, we will also measure the relative frequency at which different VH gene families are targeted for rearrangement. In this way, we will assess whether the expression of different VH genes in fetal and adult B-cells primarily reflects different outcomes of cellular selection (hypothesis-1) or the possible absence of cellular selection in fetal B-cell development (hypothesis-2). The second aim deals with the control of VL gene expression. We have observed a striking example of differential control of V-lambda-l/J-lambda-l rearrangement in two lines of IgH (mu) transgenic mice (M54 and 3H9 mice). In MM but not 3H9 mice, V-lambda-1/J-lambda-l rearrangement is suppressed. We hypothesize that the differential effect of these transgenes is due to a VH-related difference in the M54 and 3H9 mu chains or a tg-induced difference in gene expression between M54 and 3H9 B lineage cells. We will test these hypotheses using several different strategies. This will include testing whether the M54 mu chain, or any of the gene products identified by cDNA subtraction between M54 and 3H9 pre-B-cells, can inhibit temperature-induced V-lambda-1/J-lambda-1 rearrangement in transformed pm-B-cell lines.